The present invention relates to an electrical apparatus for driving an ultrasonic transducer in a surgical handpiece for the fragmentation and aspiration of tissue at an operation site on a patient, and to electronic control loops in the electrical circuitry of the apparatus. In particular, the invention relates to an apparatus for driving an ultrasonic surgical device while maintaining the vibration frequency at mechanical resonance utilizing a feedback control. The invention is also concerned with means for controlling amplitude and with a digital interface for monitoring performance and restoring normal operation when the system parameters exceed analog control loop boundaries. The invention also provides means for controlling the tissue selectivity of the apparatus.
The use of ultrasonically vibrating surgical devices for fragmenting and removing unwanted tissue by aspiration with precision and safety has led to the development of valuable surgical procedures. Initially, the technique of surgical aspiration was applied to the fragmentation and removal of cataract tissue as disclosed, for example, in U.S. Pat. No. 3,589,363. Later, similar techniques were applied with significant success to neurosurgery and other surgery specialties where the application of ultrasonic energy through a handheld device for selectivity removing tissue on a layer-by-layer basis with precise control was found to be feasible.
Certain devices known in the art characteristically produce continuous vibrations having a substantially constant amplitude at a frequency of about twenty to about thirty KHz up to about forty to about fifty KHz. U.S. Pat. Nos. 4,063,557, 4,223,676 and 4,425,115 disclose a device suitable for the removal of soft tissue which is particularly adapted for removing highly compliant elastic tissue mixed with blood. Such a device is adapted to be continuously operated when the surgeon wishes to fragment and remove tissue, and generally is operated by a foot switch.
A known instrument for the ultrasonic fragmentation of tissue at an operation site and aspiration of the tissue particles and fluid away from the site is the CUSA model System 200 Ultrasonic Aspirator manufactured and sold by Valleylab, Inc. of Stamford, Conn.; see also U.S. Pat. No. 4,827,911. When the longitudinally vibrating tip in such an aspirator is brought into contact with tissue it gently, selectively and precisely fragments and removes the tissue. Advantages of this unique surgical instrument include little damage to healthy tissue in a tumor removal procedure, blood vessels can be skeletonized, healing of tissue is promoted, no charring or tearing of margins of surrounding tissue, only minimal pulling of healthy tissue is experienced, and excellent tactile feedback for selectively controlled tissue fragmentation and removal is provided.
In many surgical procedures where ultrasonic fragmentation instruments are employed additional instruments are required for tissue cutting and hemostasis at the operation site. For example, hemostasis is needed in desiccation techniques for deep coagulation to dry out large volumes of tissue and also in fulguration techniques for spray coagulation to dry out the surface of tissues.
The apparatus disclosed in U.S. Pat. Nos. 4,931,047 and 5,015,227 provides hemostasis in combination with an ultrasonically vibrating surgical fragmentation instrument and aspirator. The apparatus effectively provides both a coagulation capability and an enhanced ability to fragment and aspirate tissue in a manner which reduces trauma to surrounding tissue.
U.S. Pat. No. 4,750,488 and its two continuation U.S. Pat. Nos. 4,750,902 and 4,922,902 disclose a method and apparatus which utilize a combination of ultrasonic fragmentation, aspiration and cauterization.
In an apparatus which fragments tissue by the ultrasonic vibration of a tool tip, it is desirable, for optimum efficiency and energy utilization, that the transducer which provides the ultrasonic vibration should operate at resonant frequency. When the transducer is a piezoelectric crystal the frequency at which it vibrates will correspond to the frequency of the electrical driving signal which causes the vibration. The operation is most efficient when the transducer vibrates at its resonant frequency. However, changes in operational parameters, such as, changes in temperature, thermal expansion and load impedance, result in deviations in the resonant frequency.
Accordingly, controlled changes in the frequency of the driving signal are required to track the resonant frequency.
The circuit disclosed in U.S. Pat. No. 4,750,488 includes a frequency control loop which depends upon a feedback signal obtained by adding two signals that are proportional to the voltage and current input to the piezoelectric transducer.
U.S. Pat. No. 4,965,532 discloses a circuit for driving an ultrasonic transducer including a frequency control means utilizing a feedback control dependent upon first and second phase detection signals.
It has now been found that an efficient frequency control is obtained with the aid of a unique control loop which includes a feedback piezoelectric crystal mechanically coupled to the piezoelectric transducer.
The use of a feedback crystal in a tuned circuit which provides a filtered signal to control a driving signal in an ultrasonic system is disclosed in U.S. Pat. No. 4,012,647. The system disclosed in this patent is not a surgical apparatus and the combination of ultrasonic vibrator, amplifier and tuning inductance with feedback from the feedback crystal to the input of the amplifier, constitutes an oscillator. In contrast thereto, the novel circuit of the present invention incorporates a voltage controlled oscillator (VCO) as part of a control loop. The feedback signal from a feedback crystal is input to the control loop which then drives the amplifier. The advantage of this novel circuit is that it tracks mechanical resonance without electrical interaction.
A problem which frequently arises during the operation of an ultrasonic surgical apparatus which includes a feedback control loop is the propensity of the control loop to lock into an unwanted adjacent frequency rather than the desired resonant frequency.
The occurrence of this problem depends upon the frequency spectrum of the system and the control loop characteristics. If the control loop is underdamped the large transient overshoots upon start-up or rapidly changing loads move the driving frequency toward the adjacent frequencies. The propensity of the control loop to lock into an unwanted adjacent frequency increases with the magnitude of control loop overshoot and the proximity of said adjacent frequencies.
Due to performance requirements and manufacturing variances, it is difficult to produce a pure analog control system which is not prone to said irregularities. Also, a difficulty in the manufacture of ultrasonic vibrators is the variation in resonant frequency due to variations in materials and manufacturing processes. Such variations in resonant frequency result in a greater magnitude of error signal in the operation of the control loop. The probability of irregularities increases in direct proportion to the magnitude of the error signal.
It has now been found that such irregularities may be avoided by the use of a microprocessor-based system interactively coupled to an analog control loop, which system provides a digital interface for monitoring performance and restoring normal operation when the system parameters exceed analog control loop boundaries.